1. Field of Invention
This invention relates generally to probe assemblies for testing integrated circuit (IC) devices whose contact pads are deployed in a predetermined pattern in a common plane, and more particularly to an assembly of this type which includes a dielectric film membrane having a planar contactor zone on which are cantilevered in a matching pattern spring contact fingers whereby when an IC device to be tested is raised to bring its contact pads into engagement with the spring fingers, this action causes the fingers to flex inwardly, and in doing so to scrub the contact pads and thereby ensure effective electrical contact therewith.
2. Status of Prior Art
Many complex electronic circuits previously regarded as economically unfeasible and impractical are now realizable in integrated circuit (IC) form. The fabrication of a single-crystal monolithic circuit involves the formation of diodes, transistors, resistors and capacitors on a single microelectronic substrate formed on a silicon wafer. The circuits are applied to the wafer by photolithography, each wafer containing an array of identical integrated sections. The wafer is then sliced into "dice," so that each die or chip carries a single integrated circuit.
In a typical IC chip, input and output terminals, power supply and other circuit terminals are created by metallized contact pads, usually deployed along the margins of the chip. The geometry of the chip is either square or rectangular, and the marginal locations of the contact pads thereon depend on the circuit configuration and the available marginal space.
In order to ensure that the various circuits in each IC chip are functioning properly, so that the chip will operate reliably in the electronic apparatus in which it is included, one must test the chip before it is installed. The packaging of an IC chip is relatively costly and time consuming, and because a significant number of IC chips fail the test and have to be discarded, it is also desirable that each chip be tested before being placed in its package.
An unpackaged IC chip having no pins or leads can be electrically connected to IC chip-testing instrumentation by means of a test probe card, one well known form of which is disclosed in U.S. Pat. No. 4,382,228 to Evans. The Evans card includes an opening providing access to the IC chip being tested, the opening being surrounded by a ring of spaced conductive platforms on which are anchored blade-like needle holders. Extending from each holder is a deflectable needle to engage a respective contact pad on the IC chip being tested.
In a subsequent Evans U.S. Pat. No. 4,719,417, the test probe card is provided with double-bent needles such that when the IC chip to be tested is raised upwardly to cause its contact pads to engage the tips of the needles, further upward movement of the chip gives rise to lateral displacement of the tips along the surfaces of the contact pads. This results in a scrubbing action serving to remove any oxide film formed on the pads that would otherwise interfere with effective electrical contact.
As pointed out in Evans U.S. Pat. No. 4,719,417, since all contact pads on the IC chip lie in a common plane and must be simultaneously engaged in order to carry out testing, it is essential that all needle tips or points lie in a plane parallel to this common plane. Consequently, a fundamental requirement of a needle probe is planarization of the needle points.
Hence a test probe card for testing an IC device must have contact probes capable of carrying out a scrubbing action to remove oxide film from the contact pads of the device, and the contact probes must also lie substantially in a common plane so that they all simultaneously engage the contact pads with substantially the same degree of pressure.
While these requirements are satisfied with a test probe card of the type shown in Evans U.S. Pat. No. 4,719,417 in which the probes take the form of double-bent needles, a card of this type does not meet a requirement often encountered in IC chips now being produced which have an ever-increasing multitude of contact pads. When confronted with a high density of contact pads on an IC chip, as a practical matter it then becomes impossible to crowd a corresponding number of needles on the test probe card without the needles touching each other and thereby rendering the test probe card defective. Furthermore, as alternating-current frequencies used in testing IC devices continually become higher, conventional needle probe arrangements which lack a grounded shield give rise to signal degradation.
In order to provide a test probe card suitable for IC chips having a high density of contact pads, the patent to Gangroth et al., U.S. Pat. No. 4,649,339, makes use of a flexible dielectric film having a sheet of copper laminated thereto that is etched to define conductive leads or traces which terminate in probe contacts. These probe contacts are patterned to engage the contact pads of high density VLSI or other integrated circuits. To bring about engagement between the probe contacts on the flexible film membrane and the contact pads on the IC chip, the film is arranged as a diaphragm covering an air chamber. When pressurized air is supplied to this chamber, the film is then flexed outwardly to conform to the surface of the IC chip. The practical difficulty with this arrangement is that the bulging film fails to effect planarization of the probe contacts thereon when they engage the IC contact pads. The absence of such planarization inhibits proper testing of the IC device. Moreover, users generally object to having any part of the device other than the probe contacts making physical contact with the wafer under test.
A similar test probe arrangement suffering from the same disadvantages is disclosed in the Brown patent, U.S. Pat. No. 4,820,976, in which gas is supplied to a pressure chamber covered by a flexible film having probe contacts thereon which when the film is subjected to pneumatic pressure bulges outwardly to cause the probe contacts to engage the contact pads on an IC chip.
Rath U.S. Pat. No. 4,758,785 discloses a motorized lift system for raising an integrated circuit device upwardly against the probe contacts on a flexible film, above which is a resilient pressure pad of silicone rubber. The practical difficulty with this arrangement is that the rubber pressure pad does not reliably ensure planarization of the probe contacts, and without such planarization, proper testing cannot be effected.
It must be borne in mind that when an IC wafer whose contact pads lie in a con, non plane is raised by a lifting mechanism toward a probe test card in which the probe contacts are formed on a flexible film, the wafer may then be caused to assume a slight angle relative to the plane of the film. As a consequence, contact pressure between the probe contacts and the contact pads will not be uniform, and this may militate against effective testing of the device.
To provide a high-density test probe assembly capable of accurately testing an IC device having a multiplicity of contact pads deployed in a common plane, the Evans et al. U.S. Pat. No. 4,975,638 discloses an assembly having a membrane formed by a flexible film of dielectric material, the membrane including a square contactor zone from whose corners extend radial slots to define suspension quadrants or wings. Probe contacts are formed in a matching pattern on the face of the contactor zone. The wings are supported at their ends on corresponding branches of a mounting frame whereby the contactor zone sags below the frame which surrounds a central port in a printed circuit board, the port exposing the contactor zone to the IC device to be tested.
In the Evans et al. '638 assembly, the probe contacts or bumps formed on the face of the contactor zone are connected to conductive traces running along the suspension wings. Since the assembly is put to repeated use in the course of which the probe bumps carry out a scrubbing action, the bumps are subject to wear. Also, they lack adequate salience. Because the dielectric film on which the bumps are formed may be imperfect and exhibit minor projections, salience is necessary to raise the bumps above these projections.
We have found, however, that contact bumps formed on the face of the contactor zone, even if given greater salience, tend to collect debris in the course of testing procedures. Moreover, compliance of the individual bumps is inadequate. Any seemingly minor or ordinary difference in adjacent bump heights require forces that are unacceptably high in order to obtain good mechanical contact on the recessive bumps by reason of insufficient elasticity in the membrane carrying the bumps.
We have also found that scrub is unreliable when limited to one direction, which is the case with devices having contact bumps formed on the face of the contactor zone. Such scrub may be achieved by various means, for example, by a cantilevered mounting arm angled downward that produces a slight lateral excursion when overdrive results in vertical excursion of the probe assembly. The net resulting lateral movement depends on the friction between the bumps and the pads on the device under test, and the bearing play found in the chuck of the testing machine. Such bearing play may range from virtually zero to dimensions that match or exceed the desired length of scrub. In the latter case, no scrub is produced.
Also of prior art interest are the following references of record in the Evans et al. patent application:
1. U.S. Pat. No. 4,891,585 (1990) to Janko et al. PA1 2. U.S. Pat. No. 4,491,391 (1990) to Greub et al. PA1 3. Japanese patent 0260861 (1985) PA1 4. Japanese patent 0286243 (1987) PA1 5. European patent 0259163 (1988) to Rath PA1 6. European patent 0304869 (1989) to Barsotti et al.